Add a `getStatus` function to root of axe

We'd like to be able to quickly determine the state of axe. This can knowing if `axe.setup` has already been called before or knowing if axe is currently running. A simple API can be a function off root that returns an object with booleans for the status:

```js
axe.getStatus() 
/*
{
  setup: true,
  running: false
}
*/
```